1. Field
The following description relates to a hyper-lens, and more particularly, to a projecting type hyper-lens for enlarging an image of the object while converting evanescent waves into propagating waves.
2. Description of the Related Art
The shape of an object can be recognized by imaging light (electromagnetic waves) scattered by the object. With regards to humans, some of the visible rays scattered from an object enter the eyeball, pass through the cornea, and crystalline lens where the visible rays are bent, and finally form an image on the retina where brightness and color of the light are sensed in cells of the retina. Optical signals obtained by sensing the brightness and color of the light are sent to the brain in a manner which allows human to recognize the object.
In general, a resolving power is a degree to which an optical system is able to discern the detail of an image. For example, when an optical system is loosely said as having a resolving power of ‘d’, the optical system is able to distinguish two objects spaced a minimum distance ‘d’ apart from each other. In optical theory, the resolving power obtainable by an optical instrument has an absolute minimum value which is half the value of the wavelength of light used in the optical device.
Accordingly, a conventional optical microscope has a resolving power of just under 200 nm corresponding to half of the shortest wavelength of visible light. If an object to be viewed has a dimension smaller than 200 nm, for example viruses or giant molecules, an electron microscope using an electron beam having a wavelength much shorter than that of visible rays is needed. However, an electron microscope is a highly complicated and expensive piece of equipment compared with an optical microscope.
As an alternative solution to the above problems, an optical microscope having an improved resolving power based on the characteristics of light has come to light. In general, the light scattered from an object has components of evanescent waves and propagating waves having different properties to each other. The propagating wave suffers low loss but offers a maximum resolution of half a wavelength. In the other hand the evanescent wave provides several times higher resolution than propagation wave but suffers very high loss. So in using evanescent wave for image formation the object should be roughly within a half wavelength from the front face of the imaging system. For example in the case of λ/2 dipole antenna, the emitted propagating wave and the evanescent wave have nearly equal amplitude at λ/6 distance from the antenna where λ is the wavelength of the radiated electromagnetic wave and the evanescent wave attenuates to 1/10 of the propagating wave around the half wave distance. FIG. 1A is a view showing characteristics of a conventional lens. As shown in FIG. 1A, a propagating wave 110 is unchanged when compared before and after passing through the conventional lens 101, but an evanescent wave 120 exhibits a significant decrease in intensity after passing through the convention lens 120.
Recently, a device known as a superlens has been developed which is capable of amplifying evanescent waves using surface plasmon [N. Fang, H. Lee, C. Sun, and X. Zhang, “Sub-diffraction-Limited Optical Imaging with a silver Superlens,” Science 308, 534, 2005.]. That is, the superlens allows evanescent waves to be used in forming an image, exceeding a resolution limit of a convention lens. FIG. 1B is a view showing characteristics of a conventional superlens. As shown in FIG. 1B, the propagating wave 110 is unchanged when compared before and after passing through the superlens 102, whereas the intensity of an evanescent wave 120 passing through the superlens 102 is amplified but quickly decays after passage.
In this regard, in order for the superlens to amplify the evanescent wave scattered from an object, a distance between a front surface of the superlens and the object needs to be roughly smaller than a half-wavelength of the light. In addition, in order for the superlens to form an image using the amplified evanescent wave, a distance between a rear surface of the superlens and an image plane needs to be smaller than a half-wavelength of the light; and since the superlens offers a real size image, electron microscope is also needed to see the subwavelength image formed by a superlens.
On the other hand, the newly introduced hyperlens is capable of forming a magnified image using an evanescent wave. If a critical size is magnified up to a certain level such as over the half wavelength it is no longer an evanescent wave but a propagating wave. A hyperlens converts evanescent wave to propagating wave in this manner. Since the propagating wave has a low decay rate, the hyperlens can form a magnified image at a location remote from a rear surface of the hyperlens. FIG. 1C is a view showing characteristics of a conventional hyperlens. As shown in FIG. 1C, the propagating wave 110 is unchanged when compared before and after passing through the hyperlens 103, and the evanescent wave 120 passing through the hyperlens 103 changes into the propagating wave 110 and thus exhibits no substantial decrease in intensity. Since the image formed by the hyperlens is far field image and larger than half the wavelength, one can recognize an object having a size several times smaller than the wavelengths of visible light using a combined system of a hyperlens and an optical microscope.
But like superlens when imaging an object using a hyperlens, the object needs to move as close as possible to the front face of the hyper-lens such that evanescent waves coming from the object are incident onto the hyper-lens before considerable decay.
Since the front face of a conventional hyperlens is a recessed shape as the inner surface of a cylinder the access of the object and the front face of the hyper-lens may be seriously limited due to the substrate contact problem. In this case, a substrate layer where a main lens layer of the hyperlens is mounted may come into contact with the substrate where the object is mounted, thus the access of the object to the hyper-lens is limited.